1. Field of the Invention
This invention relates to an agent for protecting the surface of a lithographic printing plate, especially to an emulsion type plate protective agent.
2. Description of the Prior Art
The final step of the process of making a lithographic printing plate is generally referred to as a gumming step since the plate is coated with a gumming solution to protect the non-image area (which retains water and repels the greasy printing ink). While the primary purpose of gumming is to desensitize or increase the hydrophilicity of the non-image area of the lithographic printing plate, it has other purposes. Namely, to prevent possible deterioration of the lithographic printing plate during the period between making the plate and printing or during the period when printing is interrupted; to prevent the non-image area from becoming receptive to printing ink due to sebum on fingers and other foreign matter during handling of the lithographic printing plate such as when it is set on a printing machine; and to prevent development of scratches and other flaws on the non-image area or image-area (which repels water and accepts printing ink) during handling of the plate.
Among gumming solutions of the conventional type, the most effective one is a gumming solution of an emulsion type which comprises an oil phase having dissolved in a petroleum fraction a lipophilic substance such as a lipophilic surfactant soluble in an organic solvent and an aqueous phase having a water soluble dextrin dissolved therein. When this type of gumming solution is used to gum a plate for lithographic printing, the image-area is protected by the lipophilic substance in the oil phase whereas the non-image area is protected by the water soluble dextrin in the aqueous phase. As a result, the lipophilicity of the image-area decreases only slightly even if it is in contact with the water soluble dextrin.
However, this emulsion type gumming solution is unable to completely prevent a decrease in the lipophilicity of the image area, and it is not as effective in desensitizing the non-image areas as expected. In addition, the lipophilic substance which precipitates as a result of evaporation of the petroleum fraction stains the non-image area. Furthermore, since the petroleum fraction is volatile, the gumming solution emits strong odors which is intolerable in a working environment.